There is a need for methods of measuring low humidity levels in the fields of microelectronic, communication and electro-optical systems. Relatively low levels of humidity can cause degradation in the performance of these systems and in some cases may cause irreversible damage. New communication systems should work with high reliability for long periods of time without requiring maintenance. In these systems, the ability to perform maintenance activity is limited and therefore such maintenance should be done only when necessary.
Many electro-optical systems are sensitive to humidity and therefore their maintenance includes a periodic drying operation. Such a drying operation is usually done without taking into account the precise humidity level of the system. As the technology of electro-optical systems develop towards higher energy density and lower temperature, the humidity level and contamination level is expected to decrease accordingly. When the temperature is low, water vapor can condense on the surface of an optical device, thereby forming a water droplet. This droplet may act as a lens that focuses the energy of the laser, causing physical damage to the optical device. Other examples of less severe damage which may result include inferior system performance and longer delay times.
A humidity sensor integrated into a system can be used to warn of the presence of high humidity that can be hazardous to the system. The humidity sensor may further indicate the need for drying. The humidity sensor must be small, light and reliable. There are many commercially available humidity sensors. Each type of sensor usually meets the requirements of a certain market or application. However, there is no single sensor that is suitable for all requirements and for every humidity range.
There are currently few technologies that measure low humidity levels. The most popular system is based on the diffusion of water vapor into a solid state material, such as Al2O3. This sensor acts as a capacitor. The number of water molecules absorbed determines the electrical impedance of the capacitor, which is proportional to the vapor pressure. Although such sensors are popular, systems for measuring low humidity level are very expensive. Moreover, these systems cannot be manufactured with great consistency and interchangeability. They also exhibit varying degree of aging and hysteresis (delayed response).
The most reliable system is known as chilled mirror hygrometers. The measurement in this sensor is done by directing a light beam onto a mirror using a diode laser. The mirror then reflects the light beam onto a photo detector. If the mirror is dry, nearly all of the light which has reached the mirror is received by the photo detector. However, with the presence of dew on the mirror, the reflective characteristic of the mirror changes and the light reflected off the mirror is diffused, thus reducing the amount of light reaching the photo detector. The mirror is in thermal contact with a chiller which is capable of cooling the mirror below dew point. The system is set up to find the exact temperature on the mirror at which the first dew is formed. This temperature corresponds exactly to the dew point. This method is very sensitive, but cumbersome and expensive to implement and maintain.
Similar systems are based on surface acoustic waves (SAW). These sensors utilize SAW devices to detect the presence of moisture. The SAW sensor generates surface acoustic waves which are transmitted through a delay path using the reversed piezoelectric effect. Condensation forms on the surface of the SAW device, which attenuates the wave and alters its velocity. The SAW device is cooled as in the chilled mirror technology, and the SAW temperature at which the wave changes its properties is the dew point. Such systems are extremely sensitive, but very complex and expensive.
Other piezoelectric sensors are based on Quartz Crystal Microbalance (QCM) technology. A commercial QCM system measures moisture by monitoring the vibration frequency change of a coated quartz crystal that is alternately exposed to wet and dry gas. A gas sample is divided into two streams, a sample stream and a reference stream, which are alternately passed across the measuring crystal. The crystal is coated with a polymer hygroscopic layer. Although this system is very accurate, can measure very low humidity levels, and has a fast response time, the system is also very expensive and not suitable for in-situ applications.
Another humidity sensor utilizes a coating on a restrained or non-vibrating cantilever beam. Changes in water vapor pressure cause stress in the coating and the beam. This stress is measured to provide an indication of the vapor pressure. This is a sensitive humidity sensor, but not for very low humidity levels.
There is therefore a need for a reliable humidity sensor for the lower humidity range, which is small, simple and inexpensive.